1. Field of the Invention
The present invention relates to a method of degassing a thin layer and a method of manufacturing a silicon thin film, and more particularly, the present invention relates to a method of removing a gas impurity formed in a silicon thin film during formation of the silicon thin film and a method of manufacturing such a silicon thin film on a substrate.
2. Description of the Related Art
Poly crystalline silicon (“poly-Si”) has a higher mobility than amorphous silicon (“a-Si”) and thus is applied to various electronic devices including flat panel display (“FPD”) devices, solar batteries, and the like.
In general, a material robust to heat, for example, glass, is used to obtain high quality poly crystalline silicon. Poly crystalline silicon is formed on a material such as glass robust to heat at a high temperature using an a-Si deposition method such as chemical vapor deposition (“CVD”) or plasma enhanced CVD (“PECVD”). A maximum size of crystalline particles obtained using such a conventional method is within a range between about 3000 Å and 4000 Å. It is very difficult to obtain crystalline particles having a greater size than the above maximum size, however, increasing a size of crystalline particles is important to obtain high quality poly crystalline silicon.
Methods of forming poly crystalline silicon electronic devices on plastic substrates have been studied. A low temperature process such as a sputtering method is required to form a poly crystalline silicon electronic device so as to prevent plastic from being thermally deformed. The low temperature process is required to prevent a thermal impact being applied to a substrate and a process defect occurring in a high temperature process during manufacturing of a device. Although a plastic substrate is weak to heat, it is light, flexible, and robust. Thus, the plastic substrate has been studied as a substrate of FPD devices.
In general, between 10% and 20% of the original amount of hydrogen remains in an a-Si layer formed using CVD or PECVD. The remaining hydrogen is removed through a dehydrogenation process using a furnace. The dehydrogenation process includes a heat treatment process performed at a high temperature and thus it is difficult to apply such a process to a silicon thin film formed on a plastic substrate weak to heat. Excimer laser annealing (“ELA”) is performed at a low temperature to dehydrogenate the silicon thin film formed on the plastic substrate. In ELA, a high energy laser is irradiated onto the silicon thin film for a very short time. Thus, the silicon thin film is ablated due to an evolution of impurities in the silicon film.
A sputtering method uses an inactive gas, and thus hydrogen does not remain in the silicon thin film. Instead of this, an inactive gas used as a gas atmosphere during sputtering, for example, argon Ar, helium He, xenon Xe, or the like is drawn into the silicon thin film. According to an experiment, in a case where Ar is used in a sputtering method, the silicon thin film includes Ar in an amount of between about 1% and 3% after sputtering. In a case where Xe is used in the sputtering method, the silicon thin film includes Xe in an amount of between about 1% or less after sputtering. Although the gas impurities are in a low amount, the gas impurity still badly affects a quality of the silicon thin film, and therefore a gas impurity present in a silicon thin film is required to be minimized. Thus, a heat treatment using a furnace or ELA is required. Although such a heat treatment is used, Ar is not easily removed. In an experiment, Ar is not removed at a temperature of 600° C.